The overall goal of this application is to investigate redox-regulated cellular and molecular mechanisms providing the homeostatic control of interleukin 4 (IL-4)-mediated signal transduction, gene expression, and immune responses. IL-4, which is secreted by activated Th2-lymphocytes, basophils, and mast cells, executes pleiotropic functions, including induction of Th2-differentiation, immunoglobulin class switching and proliferation of B cells, and suppression of Th1 differentiation and macrophage activation, among others. IL-4 acts as a key mediator of Th2 immunity providing protection against helminth parasite infection;however, inappropriate Th2 responses may lead to the pathogenesis of a variety of atopic diseases, including asthma and allergy. IL-4 initiates transmembrane signaling through two major pathways: the insulin receptor substrate (IRS)- phosphatidylinositol 3-kinase (PI3K) and the signal transducer and activator of transcription (STAT)6. Both the pathways are activated by tyrosine phosphorylation of the high-affinity IL-4 receptor IL-4R&#945;. Because IL-4- generated intracellular signals are limited in both magnitude and duration under physiologic condition, dephosphorylation of IL-4R&#945;is a primary mechanism for its deactivation. Previously, we and others have shown that protein tyrosine phosphatases (PTPs) SHP-1 and CD45, which are predominantly expressed in hematopoietic cells, serve as negative regulators of IL-4 signaling. Catalytic cysteine residues of PTPs, in general, are susceptible to oxidative inactivation by reactive oxygen species (ROS), including superoxide (O2 &#56256;&#56352;) and hydrogen peroxide (H2O2). Recently, we found that IL-4-stimulation of cells generated ROS via IRS-PI3Kdependent activation of NAD(P)H oxidase (NOX)1 and NOX5L. ROS, in turn, promoted IL-4 receptor activation by causing reversible oxidative inactivation of the catalytic cysteine of PTP1B that associated with and dephosphorylated IL-4R&#945;. ROS generated by other cytokines, including IL-3, tumor necrosis factor-&#945;, and erythropoietin, also promoted IL-4 receptor activation and signal transduction. In addition, we found that the anti-oxidative enzymes peroxiredoxin (Prx) II and Prx VI catalytically reduced IL-4-generated ROS, thereby attenuating the IL-4 receptor activation and subsequent signal transduction and gene rexpression. Based on these findings, we hypothesize that inactivation of IL-4 receptor-associated PTPs by cytokine-generated ROS is a physiologic mechanism for the amplification of IL-4 receptor activation in both cis and trans;on the other hand, Prx II and Prx VI alleviate oxidative inactivation of PTPs by scavenging ROS via catalytic reduction, thereby restoring the cellular homeostatic control of IL-4-dependent signal transduction, gene expression, and immune responses. To test these hypotheses, we will (i) identify the molecular targets of IL-4-generated ROS and determine the effects of ROS on IL-4-regulated gene expression and immune responses in vivo, using p22phox-knockout (KO) mice, and (ii) define the roles of Prx II and Prx VI in the regulation of IL-4-dependent signal transduction and immune responses in vivo, using Prx II-KO and Prx VI-KO mice.